Electrical energy may be applied to the spinal cord or a peripheral nerve, for example, to treat regions of the body that are affected by chronic pain from a variety of etiologies. The electrical energy is delivered through electrodes positioned proximate the nerve tissue to be stimulated, for example, external to the dura surrounding the spinal cord. The electrical energy stimulates target nerve tissue to cause a subjective sensation of numbness or tingling in the affected region of the body, known as “paresthesia.” The electrodes may be carried by either of two primary vehicles: a percutaneous lead and a laminotomy or “paddle” lead.
Percutaneous leads typically have three or more equally-spaced electrodes. They are positioned using a needle that is passed through the skin and, for example, between the desired vertebrae and onto the top of the dura. Percutaneous leads deliver energy radially in all directions because of the circumferential nature of the electrode. Paddle leads have a paddle-like configuration and typically have multiple electrodes arranged in one or more columns. Paddle leads provide more focused energy delivery than percutaneous leads because electrodes may be present on only one surface of the lead. Paddle leads may be desirable in certain situations because they provide more direct stimulation to the target nerve tissue and require less energy to produce a desired effect.
When electrical energy is applied to nerve tissue using such stimulation electrodes at an intensity above the threshold of the nerve tissue, an action potential is generated within individual nerve fibers that are stimulated. The number of nerve fibers that are stimulated is directly related to the intensity of the stimulation. The intensity of the stimulation is directly related to the distance between the stimulation electrodes and the nerve tissue and affects the level of paresthesia in the affected region of the person's body. As the person moves, the stimulation electrodes may move toward or away from the nerve tissue being stimulated, thereby increasing or decreasing the intensity of the stimulating energy reaching the nerve fibers and ultimately the number of stimulated nerve fibers. This in turn affects the level and location of paresthesia in the affected region of the person's body. In some instances, such changes in the level and location of paresthesia experienced by the person may reduce the effectiveness of the stimulation, which is typically undesirable. For example, a person typically adjusts the level of stimulation to the nerve tissue to produce a stimulation paresthesia that covers the area of pain in the person's body, thereby reducing the overall level of pain experienced by the person. When the person then sits down, stands up, bends over or makes other physical movements that may change the location of the stimulation electrode with respect to the target nerve tissue, the person may experience a paresthesia that is less than optimal for relieving the person's pain. In addition, in certain situations, the electrodes may move closer to the target nerve tissue, which may cause the person to experience additional pain in the affected region.